This invention generally relates to devices for detecting the presence of slag in a molten metal, and is particularly concerned with a slag detecting device of enhanced sensitivity and reliability for use on a ladle shroud in a facility for the continuous casting of steel.
As is illustrated in FIG. 1, in a continuous steel casting operation, the refined steel 1 is continuously poured from a ladle 3 into a tundish 5 through a pour opening 7 which may be opened or closed by a slide gate valve 12 (not shown). To prevent ambient oxygen from coming into contact with the flow 9 of liquid steel conducted from the ladle 3 to the tundish 5, a tubular shroud 11 is provided whose lower end 13 is disposed below the level 15 of steel 16 in the tundish 5. Steel poured into the tundish 5 is ultimately admitted through a second shroud 17 into a continuous casting mold (also not shown).
As a result of the previous refining processes that the steel 1 is subjected to in the ladle 3, a layer of slag 19 is built up over the upper surface of the steel 1. Ladle slag typically comprises calcium-alumino silicates, with smaller concentrations of magnesium, iron, and manganese oxides and other compounds in a molten state. While such slag 19 often serves the useful purpose of drawing out unwanted impurities in the steel (such as sulfur), it is also highly erosive to tundish refractories. Hence, it is important that the level of the steel 1 in the ladle 3 be continuously monitored so as to insure that no slag runs into the tundish 5 as the steel in the ladle is poured out. Such an unwanted flow of erosive slag can destroy the refractory lining that forms the inner surface of the tundish 5, and could contaminate the steel castings produced in the continuous casting mold.
To prevent the unwanted introduction of slag from a ladle into a tundish, several types of slag-detection devices have been developed. One such device comprises a coil through which a high frequency alternating current is passed in order to create a fluctuating magnetic field. The coil is placed near the discharge nozzle of the ladle and the tundish so that the fluctuating magnetic field it emanates can interact with the flow of molten steel. Because the magnetic permeability of slag is higher than that of molten steel, the impedance of the coil to the alternating current increases as soon as the slag is introduced into the flow of steel. Hence, the presence or absence of slag is detected by the continuous monitoring of the impedance of the coil. Unfortunately, such detectors are expensive, as it is difficult to economically manufacture such a coil-type slag detector which is capable of withstanding the elevated temperatures of approximately 1800.degree. F. in the vicinity of the discharge nozzle. Moreover, such prior art detectors have not proven themselves to be sufficiently sensitive or reliable to allow the system operator to operate the ladle slide gate valve in such a manner to consistently prevent deleterious amounts of slag from entering the tundish while maximizing steel yield.
Because of these shortcomings, other types of slag detectors were developed, one of the most advanced being disclosed and claimed in U.S. Pat. No. 5,375,816. As is schematically illustrated in FIG. 1, this slag detector 20 comprises only a steel pin 21 mounted in the tubular shroud 11 such that its inner end comes into direct contact with the flow of molten steel. The outer end of the steel pin 21 is connected to a voltmeter 23 by way of a conductive wire 25. The voltmeter measures fluctuations in the potential between the steel pin 21 and a ground. This particular type of slag detector is based upon the surprising discovery that the presence of slag in the flow of steel generates a measurable increase in the electrical potential between the pin 21, and a ground. In contrast to coil-type slag detectors, this detector 21 is extremely simple and rugged in structure, and has proven to be, on the whole, at least as sensitive to the presence of slag as coil-type sensors.
However, despite the overall improvement that such conductive pin-type slag detectors represent, there is still a need for a slag detector having the simplicity and durability of such detectors, but greater sensitivity and reliability so as to allow the operator of the steel making facility even more time in which to react to prevent significant amounts of slag from flowing from a ladle to a tundish during a pouring operation.